Transient elastic waves can be generated in a region of a material which experiences abrupt changes in stress or strain. This phenomenon is known as acoustic emission and is generally detected by means of ultrasonic transducers coupled to the material. Growth of microcracks, interfacial bond failure in fibre composite materials, and delamination of a thin film are typical examples of events which produce acoustic emission, although the acoustic radiation details generally depend on the particular mechanism involved.

Acoustic emission frequencies are usually in the range 150 - 300 kHz, that is, well above the frequency of audible sound. This permits all ambient noise to be filtered out, to leave only the frequencies of interest.

In the NanoTest and MicroTest, acoustic emission is detected by means of a piezoelectric transducer which is mounted in such a way that its resonant frequency is in the range of the signal of interest (> 200 kHz). The transducer element is mounted inside the specimen holder. This is considered superior to the alternative approach of mounting the transducer on the diamond holder since some acoustic emission events can occur during diamond unloading, when the diamond may not be effectively mechanically coupled to the specimen.

The main system components are shown schematically. The bandpass amplifier eliminates ambient noise, mains interference and very high frequency noise.

Because acoustic emission signals are generally very fast, the signal is fed through a threshold detection and integration unit which produces an analogue pulse proportional to the energy content of the sensor output voltage. The voltage level at which a signal is accepted (that is, the system sensitivity) is set prior to data acquisition.

The example shows acoustic emission from a silicon surface during a MicroTest scratch test. Since this was a low load measurement, a relatively sharp 120o conical diamond was used. This had a tip radius of approximately 20 µm. In this particular type of measurement, the applied force is increased linearly (read from the right-hand axis), and the acoustic emission is monitored continuously. A critical load for scratching (or adhesion failure in the case of a thin film) is manifested through a burst of acoustic emission.